An alarm device applied to a portable electronic device is provided. The alarm device includes a detection unit and a connecting unit. The detection unit includes a first data transfer pin, and the first transfer pin is preset to a first electric potential. When the portable electronic device is connected to the detection unit via the connecting unit, the first data transfer pin is connected to a second electric potential via the connecting unit. The detection unit is set in a detection mode. In the detection mode, when electric potential of the first data transfer pin is switched from the second electric potential to the first electric potential, the detection unit issues an alarm.